1. Field of the Invention
The present invention relates to an AC motor driving circuit which includes an AC power supply and a DC power supply capable of feeding and absorbing electric power and which drives an AC motor, and an electric car driving circuit using the AC motor driving circuit.
2. Description of the Related Art
For an AC motor driving circuit of this kind, there is previously known a circuit shown in FIG. 8 (e.g. JP-A-2005-318731).
The circuit uses a matrix converter 38 to achieve power conversion between motor generators MG1 and MG2. The matrix converter 38 is composed of nine switches arranged as a three by three switch matrix (switches SAa to SCc). A storage battery 12, a chopper 32 and a capacitor C2 form a DC power supply capable of feeding and absorbing electric power to the outside. A voltage source inverter 36 is provided to exchange electric power between the DC power supply and the motor generator MG2. The voltage source inverter 36 uses six arms, which have semiconductor switches Q31 to Q36 and diodes D31 to D36 back-to-back connected to the semiconductor switches Q31 to Q36 respectively. The six arms are composed of two U-phase bridge arms 36U, two V-phase bridge arms 36V, and two W-phase bridge arms 36W. The chopper 32 is formed of a reactor L and switching devices Q11 and Q12 paired with their respective corresponding diodes D11 and D12. A control apparatus 40 controls the AC motor driving circuit based on control signal from a controller 48.
For power conversion between the motor generators MG1 and MG2, the matrix converter 38 is operated and a switch S1 provided between the DC power supply and the voltage source inverter 36 is turned off. On the other hand, for use of both electric power from the DC power supply and electric power generated by the motor generator MG1 for driving the motor generator MG2, the on/off state of the switch S1 is controlled in a time division manner.
If an input line voltage of the motor generator MG2 exceeds an output voltage of the DC power supply in a period when the matrix converter 38 is operated to feed electric power from the motor generator MG1 to the motor generator MG2, a current flows into the DC power supply side from the motor generator MG2 side through the diodes D31 to D36 in the voltage source inverter 36. As a result, there is a possibility that the motor generator MG1 cannot perform desired drive control of the motor generator MG2. Therefore, the switch S1 is opened (turned off) in advance to avoid such a problem. On the other hand, a matrix converter for directly converting AC power into AC power with another frequency has been disclosed, for example, in JP-A-2005-269805.
In the circuit shown in FIG. 8, it is necessary to always apply a current to the switch S1 for power conversion between the DC power supply and the AC motor generator MG2. Hence, there is a problem that loss occurring due to the switch S1 results in lowering of efficiency in power conversion between the DC power supply and the AC motor generator MG2.